Add API v1.1 support

Support v1.1 of Twitter's API. This includes adding a new endpoint,
updating the rate limit headers, and bumping the version number.

Tests have also been updated.

Note that there are no checks to make sure you're using OAuth and JSON
with v1.1, so you can shoot yourself in the foot by using v1.1 with
basic auth or a non-JSON format if you want.